The present invention relates to tomographic (or sectional radiographic) equipment being adapted to inspect and/or analyze defects in a given object, particularly relating to X-ray or .gamma.-ray tomographic testing apparatus for nondestructive testing of size, dimensions and/or inner defects in industrial products.
Tomographic equipment called as a "computerized tomography scanner (CT scanner)" may be utilized to safely and accurately inspect the inner defects, tissues, construction, etc. of materials. Such a CT scanner is provided with a radiation source and radiation detector. The radiation source generates a fan beam X-ray which is spreaded along a two-dimensional sector plane. The radiation detector is opposed to the radiation source with a slice of an object to be inspected therebetween. The detector is formed with a plurality of radiation sensors which are arranged around the radial directions of the spreaded X-ray sector plane. When the source-detector configuration is rotated 360 degrees around the object in a steps of one degree, a large amount of data representing X-ray absorption of the object slice for respective angles of 360 degrees is obtained.
A tomographic image corresponding to the obtained X-ray absorption data is reconstructed by means of computer processing. The reconstructed tomographic image of respective portions in the inspected object slice may have a thousand gradations, and therefore, a precise inspection or analysis of the object material can be achieved.
In recent years, it has been proposed that a CT scanner be applied to nondestructive testing of size, dimensions and/or inner defects of industrial products. Such a CT scanner may have a configuration as shown in FIG. 1. (A similar configuration is disclosed in FIG. 1 of U.S. Pat. No. 4,293,912 issued on Oct. 6, 1981).
According to the configuration of FIG. 1, a main body 1 of the scanner has an X-ray source 2. Source 2 radiates a fan beam X-ray FB for each projection within a given spreading range. A radiation detector 3 is opposed to X-ray source 2. Detector 3 includes a large number of tiny radiation sensors which are arranged around the radial directions of the spreaded X-ray sector plane. Each of the radiation sensors senses, with a certain spatial resolution, the intensity of X-ray from source 2. The radiation path defined between source 2 and each of the radiation sensors is called an X-ray path. Each of the radiation sensors delivers an individual signal which indicates the intensity of an X-ray on the corresponding X-ray path.
Scanner main body 1 is provided with a rotation actuator (not shown). X-ray source 2 containing an X-ray tube is mounted on the rotation actuator so that the rotation center of the actuator coincides with the center of a tomography region. The actuator serves to effect a single-way rotary scanning of the X-ray. The rotary scanning angle is sequentially changed by prescribed degrees. An object material 4 to be inspected is placed within the tomography region. An X-ray radiation control, a current and voltage control for the X-ray tube, etc. are performed by an X-ray controller 5. A rotation control for the rotation actuator is achieved by a scanner controller 6. The operation of controllers 5 and 6 is governed by a system controller 7. System controller 7 also governs the whole operation of the CT scanner. Various instructions and/or data required by the system controller 7 are obtained from a console 8. An operator of the scanner may input specific data or instructions to controller 7 through a manipulation of console 8.
Respective outputs E3 (analog) from the radiation sensors of detector 3 are supplied to a data collector 9. Collector 9 includes an A/D converter. According to a control command I7 from system controller 7, the A/D converter converts the analog outputs E3 into digital X-ray absorption data D9 for each projection. Absorption data D9 is supplied to a preprocessor 10. Under the control of system controller 7, data D9 is variously processed through a log-converter, gain corrector, off-set corrector and so on contained in preprocessor 10.
Processed data D10 from preprocessor 10 is convoluted by a convolver 11 upon receipt of command I7 from system controller 7. Convoluted data D11 from convolver 11 is supplied to a back projector 12. In back projector 12, data D11 is back-projected along the projection direction, and reconstruction of a tomographic image of the back-projected data is achieved. Reconstructed image data D12 of the back projection is stored in an image memory (RAM) 13. Data D13 read-out from memory 13 is supplied to an image converter 14. In converter 14, data of a desired range of CT values contained in data D13 (or data being defined in accordance with the degree of X-ray absorption) is image-converted, so that data D14 representing the desired CT range is displayed with various white levels in a monochrome display screen of a CRT display 15.
The CT scanner of FIG. 1 will operate as follows. First, in order to obtain a tomographic image of object 4, an operator of the scanner manipulates the key board of console 8 so that the CT scanner starts to operate. Then, system controller 7 instructs the scanner controller 6 to perform the step rotation of the rotation actuator with a given angle. System controller 7 also instructs the X-ray controller 5 to perform an intermittent application of a given voltage and current to the X-ray tube for each of the step rotations. The period of intermittent voltage and current applications to the X-ray tube for each step rotation is prefixed. By the intermittent application of voltage and current to the X-ray tube, X-ray source 2 sequentially generates pulsate fan beam X-rays FB.
Object 4 is located at the rotation center (tomography region) of the rotation actuator, and X-ray source 2 faces the detector 3 through the rotation center. Accordingly, as the rotation actuator rotates, a specific slice of object 4 is subjected to the radiation of fan beam X-rays FB from various directions. Then, X-ray transmittances of respective X-ray paths for each fan beam X-ray FB are sensed by the radiation sensors of detector 3, and information of the sensed transmittances is converted to outputs E3.
The data regarding the converted outputs E3 is collected by data collector 9. For each of tomographic projections, data collector 9 supplies the collected data D9 to preprocessor 10, so that the collected data D9 is log-converted, gain-corrected, off-set-corrected, etc. Preprocessed data D10 from preprocessor 10, which indicates X-ray absorption of respective X-ray paths for each projection, is convoluted in convolver 11. Convoluted data D11 from convolver 11 is supplied to back projector 12 in which a back-projecting operation is effected. Then, CT values for the locations of respective pixels of an image are obtained, and a tomographic image corresponding to the obtained CT values is reconstructed.
The reconstructed tomographic image is stored in memory 13. In accordance with specific instructions from console 8, the gradation of the CT values regarding a desired region of the stored tomographic image is determined by image converter 14, and the tomographic image with the determined gradation is displayed at CRT display 15. Thus, the reconstructed tomographic image is displayed on the monochrome display screen with given various white levels.
Generally speaking, according to the CT scanner as shown in FIG. 1, about 300 to 600 sets of projection data are needed for an accurate inspection or analysis of industrial products. Namely, tomographic measuring is performed 300 to 600 times for each one rotation of the rotation actuator in scanner main body 1. From this, a long scanning time (5 to 10 seconds or more) is required for each inspection.
Even where the CT scanner of FIG. 1 is used for a relatively simple inspection of industrial products (e.g., a test for merely judging whether or not the inspected product is good), at least a hundred sets of projection data will be required. Accordingly, if such a CT scanner is used for inspecting the inner defects of products to be mass-produced, a smooth, fast flow of mass-produced products in the factory is unobtainable because of the length of time needed for inspection. This is one of the problems to be solved.
Further, in a medical X-ray CT scanner, a reference material is often used for a comparable measurement. In such a comparable measurement, in a preceding step, projection data for a water fantom (reference sample) is obtained for various projection directions. An error due to the characteristic of the CT scanner is detected from the projection data of the water fantom, and values for compensating the error are calculated. Next, in a measuring step, each of inspection materials is set at the tomography region in place of the water fantom, and the obtained projection data for each inspection material is compensated in accordance with the calculated error-compensating values. Then, the compensated projection data is convoluted and back-projected to obtain a reconstructed tomographic image of the inspection material (inspection sample).
Although the above reconstructed tomographic image actually indicates the sectional view of the inspection sample, it is not evident directly from this tomographic image which portion of the inspection sample deviates from the corresponding portion of the reference sample, or it is not evident directly from this tomographic image whether or not the inspected sample is a good one. This is another problem to be solved.